Neuronal function and behavior may be determined, among other mechanisms, by the storage and release of neurotransmitters and hormones. The release of these substances, which are synthesized in the cytosol, occurs by a process of exocytosis, in which a secretory vesicle fuses with the plasma membrane. When an excitable cell is stimulated, the contents of one or more vesicles are released completely into the extracellular space. Release from a single vesicle has been termed quantal release. Techniques used to study quantal release include carbon-fiber amperometry, patch amperometry, and amperometry using microfabricated planar microelectrodes. These amperometric techniques can detect quantal release of oxidizable transmitters, such as the catecholamines epinephrine and norepinephrine, using a carbon fiber or platinum microelectrode that is positioned close to the cell surface and held at a potential that is sufficiently high to oxidize the released molecules. Upon release from a vesicle, the catecholamine molecules that diffuse to the surface of the electrode are rapidly oxidized, resulting in the transfer of two electrons to the electrode. The oxidation of the molecules released in a single quantal event thus generates a transient oxidation current with a duration of a few milliseconds.